1. Field of the Invention
The present invention relates to a method of characterizing the transmission losses of an optical system and a method of measuring properties of a substrate.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Sensors are known for inspecting substrates during or after lithographic processes. For example, after a resist on a substrate is developed (having been exposed by a patterned beam of radiation, for example) a measurement and inspection step may be performed. This is referred to as “in-line” because it is carried out in the normal course of processing substrates used in production. This may serve two purposes. Firstly, it is desirable to detect any areas where the pattern in the developed resist is faulty. If a sufficient number of dies on a substrate, namely portions of the substrate that will be used to form an individual device are faulty, the substrate can be stripped of the patterned resist and re-exposed, hopefully correctly, rather than making the fault permanent by carrying out a subsequent process step, for example an etch, with a faulty pattern. Secondly, the measurements may allow errors in the lithographic apparatus, for example illumination settings or exposure dose, to be detected and correct for subsequent exposures. However, many errors in the lithographic apparatus cannot easily be detected or quantified from the patterns printed in resist. Detection of a fault does not always lead directly to its cause. Thus, a variety of “off-line” procedures for detecting and measuring errors in a lithographic apparatus are also known. This may involve replacing the substrate with a measuring device or carrying out exposures of special test patterns, for example at a variety of different machine settings. Such off-line techniques take time, often a considerable amount, during which the end products of the apparatus will be of an unknown quality until the measurement results are made available. Therefore, in-line techniques, ones which can be carried out at the same time as production exposures, for detecting and measuring errors in the lithographic apparatus, are preferred.
Scatterometry is one example of an optical metrology technique that can be used for in-line measurements of CD and overlay. There are two main scatterometry techniques:                1) Spectroscopic scatterometry measures the properties of scattered light at a fixed angle as a function of wavelength, usually using a broadband light source such as Xenon, Deuterium, or a Halogen based light source. The fixed angle can be normally incident or obliquely incident.        2) Angle-resolved scatterometry measures the properties of scattered light at a fixed wavelength as a function of angle of incidence, often using a laser as a single wavelength light source or a broadband source in combination with a wavelength selection device such as a narrow band interference filter, a dispersive prism or a diffraction grating.        
The structure giving rise to a reflection spectrum is reconstructed, e.g. using real-time regression or by comparison to a library of patterns derived by simulation. Reconstruction involves minimization of a cost function. Both approaches calculate the scattering of light by periodic structures. The most common technique is Rigorous Coupled-Wave Analysis (RCWA), though light scattering can also be calculated by other techniques such as FDTD or Integral Equation techniques. Such angle-resolved scatterometry is described in more detail in U.S. Patent Application Publication 2006/0033921 A1.
Scatterometry can also be used to inspect features formed after an etch process (which may, for example, have been controlled by a pattern formed by a lithographic process) or to measure the thickness or properties of one or more layers of material formed in a stack.
In order to perform angle-resolved scatterometry measurements, the substrate is illuminated with radiation. In order to obtain the most information from the measurements, the radiation illuminating the substrate may be linearly polarized. To obtain a high resolution a lens with a high numerical aperture (NA) is used. Preferably the NA has a value of at least 0.95. The high aperture lens is rotational symmetric but has polarization-dependent transmittance. FIG. 6 shows the pupil plane of the high aperture lens and as can be seen the transmissions at each point can be broken down into an S component and P component. In general S polarized light suffers higher transmission losses than P polarized light which can lead to a net distortion of the measured angle-resolved scatterspectrum.